1. Field of Invention
This invention is in the field of Synthetic Aperture Radar (SAR) imaging where some of the pulses making up the image are missing.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform for a high resolution image.
The plurality of returns creating a SAR image generated by the transmitted pulses along a known path of the platform make up a frame length. During the frame length, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the frame length (aperture).
The plurality of pulses transmitted during an SAR frame length, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the “length” traveled by the antenna during the frame length. The clarity of a SAR image is in many respects dependent on the quality of the motion compensation applied to each radar return prior to SAR image computation as well as on the existence of full information descriptive of the SAR image, i.e. a complete set of radar returns. The SAR process depends on the coherent, phase accurate summing of all radar returns expected within a frame. Missing pulses during the acquisition of the image introduce distortions over the whole SAR image, such as increased sidelobes, thereby reducing the SAR image clarity.
The Range Migration Algorithm (RMA), a popular way to focus the radar returns forming a SAR image, is derived assuming the radar returns arrive at a generally fixed, known rate, determined by the pulse repetition rate and/or the analog to digital conversion rate. If radar returns descriptive of the SAR image are not regular in their arrival, certain functions within the processing, such as FFTs, cannot function as expected and the image becomes blurred.